Avoid assigning duplicate RAFT IDs to new nodes.

[martinmr]

Currently, zero uses the MaxRaftId to assign RAFT IDs to new nodes. However, the
MaxRaftId value is not immediately updated and the lock is released in between
creating the proposal and sending the proposal to the RAFT node. This can lead
to situations where more than one new node is assigned the same ID.

To solve this, this change introduces a new field to generate the IDs that is
updated immediately, thus preventing multiple nodes from being assigned
the same ID.

Fixes #5436

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This change is 

[manishrjain]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @manishrjain, @martinmr, and @vvbalaji-dgraph)

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dgraph/cmd/zero/raft.go, line 217 at r1 (raw file):

	}

	if has && m.Addr != member.Addr {

This could avoid an address change from happening.

[martinmr]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @manishrjain and @vvbalaji-dgraph)

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dgraph/cmd/zero/raft.go, line 217 at r1 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

This could avoid an address change from happening.

Done. Reverted this change.

addressed comments

[manishrjain]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @manishrjain, @martinmr, and @vvbalaji-dgraph)

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dgraph/cmd/zero/zero.go, line 500 at r2 (raw file):

		if m.Id == 0 {
			m.Id = s.nextRaftId
			s.nextRaftId += 1

A proposal could error out, and then get applied later. Therefore, it is best to keep incrementing nextRaftId.

[martinmr]

Reviewable status: 0 of 2 files reviewed, 1 unresolved discussion (waiting on @manishrjain and @vvbalaji-dgraph)

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dgraph/cmd/zero/zero.go, line 500 at r2 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

A proposal could error out, and then get applied later. Therefore, it is best to keep incrementing nextRaftId.

Done.